Subsurface traveling wave antenna

ABSTRACT

A subsurface traveling wave antenna in which a first and a second insulated matched load terminated linear radiating element is positioned in a plane normal to and under the surface. Each of the radiating elements are spaced apart but are located within the refracting cone at the surface interface. The load terminated ends are located at the surface end of each element. The distance between the elements is sufficiently close to provide phase reinforcement. Means are provided for mutually exciting the radiating elements, as well as for matching the phase velocity of an electromagnetic wave propagated along the radiating element to the phase velocity of an electromagnetic wave propagating in a medium external to the radiating elements.

United States Patent [151 3,680,133

Tsao et al. [451 July 25, 1972 [54] SUBSURFACE TRAVELING WAVE 2,208,749 7/1940 Cork et a1 ..343/735 ANTENNA 3,346,864 10/1967 Harmon ..343/7 1 9 [72] Inventors: Carson K. H. Tsao, Braintree; Joseph T. Primary ExaminerEli Lieberman De Bettencourt, West Newton; Howard J. Attorney-Harold A. Murphy and Joseph D. Pannone Rowland, Newton Highlands, all of Mass.

[73] Assignee: Raytheon Company, Lexington, Mass. [57] ABS CT I I A subsurface traveling wave antenna in which a first and a [22] Flled' June I970 second insulated matched load terminated linear radiating ele- [21 Appl. No.2 48,811 ment is positioned in a plane normal to and under the surface. Each of the radiating elements are spaced apart but are Related l Dam located within the refracting cone at the surface interface. The

load terminated ends are located at the surface end of each element. The distance between the elements is sufiiciently 52 us. CI ..343/719, 343/739, 343/751, Phase reinfmemem- Means are Pmvided 343/853 mutually exciting the radiating elements, as well as for lnt.Cl. ..H0lq1/04 matching the Phase velocity of electmmagmfic Wave propagated along the radiating element to the phase velocity [63] Continuation of Ser. No. 742,690, July 5, 1968.

[58] Field of Search ..343/719, 735, 736, 739, 740,

343/751, 8 5 3 of an electromagnetic wave propagating 111 a medium external to the radiating elements.

[56] References Cited w M g H C aim Figures UNITED STATES PATENTS 1,429,240 9/1922 Hanson et al.

"u" ANTENNA CONFIGURATION PATENTED- I972 3.680.133

sum 1 or 2 x AIR I I 10 I /b v zb 3 3 L /4 '4 "U"ANTEN V ON A /a/ FEEDLINE F/GZ "V"ANTE A CONFIGUR ON v L MATcRme TRANSFORMER INVENTORS HOWARD J. ROWLAND CARSON K.l-I. TSAO JOSEPH 7. d8 BETTENOOURT BY 6M4 ATTORNEY P'A'TENTEDJULZS 1 12 Y E 3,680.1 as sum 2 OF 2 RELATIVE GM m da 0.0a 0.: Lo I0 I00 FREQUENCY m MEGAHERTZ g =RELATIVE DIELECTRIC CONSTANT 2/60 9 SUBSURFACE MEDIUM AND AIR smgvl em (DIPOLE) o 0.1 v m I0 I00 FREQUENCY m MEGAHERTZ INVENTORS HOWARD J. ROWLAND CARSON K. H. T804 JOSEPH 7. deBETTENCOURT ATTORNEY SUBSURFACE TRAVELING WAVE ANTENNA This is a continuation of application Ser. No. 742,690, filed July 5, 1968.

BACKGROUND OF THE INVENTION This invention relates to subsurface antennas, and more particularly to highly efficient buried antenna systems which may be rendered immune to military bombardment.

In survivable communications, such as at missile sites, it is anticipated that the requirements to survive in the event of nuclear attack would indicate the use of subsurface antennas for radio communications. It is well known that buried antennas near the surface may transfer and receive energy by three mechanisms. The first mechanism is the generation of a surface wave, which is a substantially vertically polarized wave propagating along the earths surface. In the second mechanism, one buried antenna generates a space wave which upon reflection may communicate with another buried antenna in the so-called up-over-and-down propagation scheme. The third transmission mechanism is directly through the rocklin which waves are polarized in directions parallel to the buried antennas. Reference is made in this regard to the Proceedings of the Conference of the British Institute of Electrical Engineers, 8-l0 November 1967, at pages 313-317, in an article entitled, Subsurface Radio Communications," by Tsao and'deBettencourt and "Progress in Radio Science," by J. T. deBettencourt in the International Radio Scientific Union (URSI), Berkeley, California, 1967, part 1, pages 697- 767.

One scheme for improving the transmission efficiency and directivity ofa subsurface antenna is to utilize a traveling wave antenna arrangement. The antenna may comprise an insulated linear radiating element which terminates in a matched load. Reactive impedance elements can be used to interconnect portions of the radiating element at periodic intervals along its extent. This provides speed matching between the phase velocity of an electromagnetic wave propagating down the radiating element and an electromagnetic wave propagating, for example, on the surface. Such a structure is disclosed in US. patent application Ser. No. 742,074, filed on July 2, 1968, entitled, A Subsurface Traveling Wave Antenna," by C. K. H. Tsao and .I. T. deBettencourt, now abandoned.

For transmission fromand reception to a buried antenna using a surface wave or through the rock" mechanism a plurality of capacitive elements are distributed along the linear radiating element. In the first case, the antenna is placed substantially parallel to the surface while in the second case the antenna is substantially normal. In the up-over-and-down" mode a plurality of inductive impedance elements disposed along a vertically placed subsurface linear radiating element are used.

The linear radiating element, which may be represented as a dipole, exhibits a radiating pattern in which most ofthe energy goes up into the rock and becomes internally reflected. Such losses due to internal reflection are to some degree reduced when the dipole is placed parallel to the surface.

It is accordingly an object of this invention to improve the gain of a linear subsurface antenna, and more particularly to improve the gain of a subsurface antenna of the traveling wave type.

In survivable communications, subsurface antenna installations are often constructed in bed rock. It is well recognized that such antennas may more easily be constructed by drilling a hole normal to the surface and inserting a linear antenna element therein.

It is accordingly another object of this invention to devise a vertical insulated linear radiating antenna of the traveling wave type which may have high gain and not be required to achieve the gain through horizontal subsurface placement.

SUMMARY OF THE INVENTION The foregoing objects of this invention aresatisfied in an embodiment in which a first and second insulated matched load terminated linear radiating element is positioned under the surface and spaced apart within the refracting cone at the surface interface. The distance between the radiating elements is such as to provide phase reinforcement if the elements are mutually excited. Means are also provided for matching the phase velocity of an electromagnetic wave propagating along each of the radiating elements to the phase velocity of an electromagnetic wave propagating in a medium external to the radiating element.

The increased gain, particularly for the vertically disposed antennas, achieve its effect through the mutual excitation of a radiating element pair spaced within the reflecting cone. This assures that substantially more of the energy will be transmitted through to the medium of propagation, such as air or space, than either a single vertically placed dipole or a horizontally placed dipole.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a pair of vertically spaced radiating elements in a U antenna configuration according to the invention;

FIG. 2 shows a pair of radiating elements mutually stimulated in a V antenna configuration according to the invention;

FIGS 3A and 3B are graphs of relative modified power gain of V antenna as a function of frequency.

DESCRIPTION OF THE PREFERRED EMBODIMENTS If an energy ray hits a surface at less than a predetermined angle, then all the energy is reflected. This is termed perfect reflection. This is best illustrated when a light ray passes through media of different optical densities. Ifa light ray goes from a more dense to a less dense optical medium in a direction normal or perpendicular to the interface, then substantially all of the energy is transferred. If the angle is either greater than or less than then internal reflection take place. Actually, there is a cone angle centered about the nor-.

mal to an interface in which substantially all ofthe energy gets through from the more dense to the less dense medium. Outside of this cone angle, there is substantially total internal reflection.

Applying these principles to the case of the buried antenna and as previously described, it is apparent that the radiating patterns of both vertically and horizontally disposed buried dipoles would lie outside of the refracting cone angle.

This may be seen in FIG. 1. In this figure, a first and second insulated linear radiating element 9 and 10 are shown vertically placed below the surface within the refracting cone angle 0 as measured from the normal at a point at the interface between the ground and air.

Each radiating element 9 and 10 may be formed from an RG/U-54 coaxial cable utilizing the center conductor 3 as the radiating element with the outer conductor and the neoprene ackets removed. A first and second plurality of inductances 2a and 2b are respectively inserted at periodic intervals along the extent of each radiating element 3. An insulating dielectric 4 surrounds each of the inductively loaded antennas. The surface end of each antenna is terminated in a matched load impedance la and lb respectively. The other end of-each antenna is coupled over line 6 to an energy source or receiver 5. The antennas are spaced apart by a distance D and each have a length L.

The propagation constant k, of each antenna is related to the phase constant B, and the attenuation constant a, by k, B, ja,. Similarly, the propagation constant of a medium external to the antenna, such as air or ground, k is related to its phase constant B, and attenuation constant a by k B ja The vertical traveling wave antenna in the ground excites a field with both vertically and horizontally polarized components. If the horizontal component is to contribute to the surface or ground wave, for example, then the contributions to the ground or surface waves from the U antenna configuration will be from the vertical traveling wave radiators and from the horizontal feed line. If we assume B, is equal to B a, (1

and a L 1, then reference is made to propagating a surface or space wave. In this context, the useful length of antenna L may be approximated by B D/a An incident horizontally polarized surface or space wave, upon being refracted into a ground wave will suffer a refraction loss p approximately equal to 20 log |k /k,| Likewise, a vertically polarized wave, upon being refracted into the ground will suffer a refraction loss p,.,. approximately equal to 20 log [k /k,|

The V antenna shown schematically in FIG. 2 has its radiating arms separated by an apex angle 20. The arms lying along axis 7a and 7b are placed in a vertical plane below the surface of the ground. Generator is coupled to the apex ends through matching transformer 8 for exciting a traveling wave which propagates toward the surface. In this embodiment, as is also shown in FIG. I, each radiating element terminates in a matched load In and lb. at the surface end thereof. The radiation field is essentially horizontally polarized. The adjustment of the phase constant may be used to maximize radiation toward the interface.

When the V antenna is excited, each infinitesimal segment of ;each radiating element may be considered a horizontal dipole contributing to the radiated wave on the surface. The V antenna is therefore equivalent to an array of horizontal dipole elements arranged vertically. If these horizontal dipole elements are to contribute to the wave field constructively, then the dipole current must be properly phased. This requires that the wavelength on each radiating element have a vertical projection equal to the wavelength in the medium.

FIGS. 3A and 3B are graphs of the relative gain of the V antenna to an insulated dipole plotted as a function of frequency. FIG. 3A shows this relationship where the relative dielectric constant e /q, of the subsurface medium with respect to air is taken at 9: In FIG. 3B the relative dielectric constant is taken at 25. The dipole is half-wave, horizontal, insulated and open circuited. The dipole is placed at the same depth in the subsurface as the depth of the upper ends of the V antenna. The total length of the dipole is equal to the maximum width W for the V.

In the ground, the reduction in power density and the efficiency of an antenna have lead to the use of the figure of merit called modified power gain. The power gain at any antenna in air may be defined as the ratio of the power density at a receiver at a distance R from the transmitting antenna to the power density in the antenna, this ratio being multiplied by 411-11 For the gain measurement in the ground, the gain in air is multiplied by e where a is the attenuation constant.

Now, the power density at the receiver is Ke Thus, the

modified power gain 6,, may be considered independent of the distance in the rock and may be expressed as cmK47TR power in antenna The relative modified power gain between the V antenna compared to the halfwave dipole was measured for a vertical projection h of 50 meters and 150 meters for a conductivity of and 10 mhos per meter. FIGS. 3A and 3B show that at 500 kilohertz the V antenna is of acceptable performance only if the relative dielectric constant is in the range of and the length of the antenna is large. The V antenna is, however, excellent at frequencies above 50 megahertz. It should be noted in passing that the optimum antenna projection h varies with frequency. At the higher frequencies a smaller )1 is required.

For purposes of FIGS. 3A and 3B the half-wave dipole was made of RG218/U coax cable. The V antenna had an insulation of radius 0.075 meters and a relative dielectric constant of LOO. The inner conductor had a radius of 0.003 meters.

In summary, an improved subsurface traveling wave antenna has been described. This antenna comprises a first and second insulated linear radiating element positioned in a plane normal to and under the surface. The surface end of each radiating element has been terminated in a matched load. Each of the radiating elements is spaced apart by W located within the refracting cone at the surface. Means are provided for mutually stimulating the radiating elements, as well as for matching the phase velocity of a wave propagating along the element to the phase velocity of a wave in a medium external to the radiating elements. A relationship between the useful radiating element length and space has been shown with reference to the wavelength in the propagation medium.

We claim: 1. A subsurface antenna for transmission and reception of electromagnetic wave energy comprising:

first and second insulated matched load terminated radiating elements positioned under the surface in a generally vertical position, the distance between the radiating elements is such as to provide phase reinforcement; means for mutually exciting said radiating elements; and means for matching an electromagnetic wave propagating along said radiating elements to the phase velocity of an electromagnetic wave propagating in a medium external to the radiating elements. 2. A subsurface antenna according to claim I, wherein: said radiating elements lie along axes which intersect to form an angle at one of their ends. 3. A subsurface antenna according to claim 2, wherein: said radiating elements are linear. 4. A subsurface antenna according to claim I, wherein: said radiating elements are linear. 5. A subsurface communications antenna comprising: first and second insulated matched load terminated linear radiating elements positioned normal to and under the surface, the elements being spaced apart by distance D, the useful radiating element length L being approximated by B- D/a when B B a a and a L l, where a and a are the attenuation constants of the radiating element and propagation medium respectively, B and B are the phase constants of the radiating element and propagation medium respectively; means for mutually exciting the radiating elements; and means for matching the phase velocity of an electromag netic wave propagating along said radiating elements to the phase velocity of an electromagnetic wave propagating in a medium external to the elements. 6. A subsurface communications antenna according to claim 5, wherein:

each radiating element propagation constant k, is related to the attenuation constant a, and phase constant B, by k, B ja,, the spacing between the radiating elements being such that IkDI s A /2, where A is the wavelength of the electromagnetic wave in the propagation medium. 7. A subsurface antenna for transmission and reception comprising:

first and second insulated matched load terminated linear radiating elements positioned under the surface, the radiating elements being spaced apart within the refracting cone at the surface interface; means for mutually exciting the radiating elements; and means for matching the phase velocity of an electromagnetic wave propagating along the radiating elements to the phase velocity of an electromagnetic wave propagating in a medium external to the radiating elements. 8. A subsurface antenna for transmission and reception comprising:

first and second insulated linear radiating elements spaced apart and defining a plane under the surface within the refracting cone of the surface interface, each element having one end near the surface terminated in a corresponding matched load; means for mutually exciting the radiating elements; and means for matching the phase velocity of an electromagnetic wave propagating along the radiating elements to the phase velocity of an electromagnetic wave propagating in a medium external to the elements. 9. A subsurface antenna according to claim 8, wherein: the first and second insulated linear radiating elements are positioned normal to the surface. 10. A subsurface antenna according to claim 8, wherein:

the first and second insulated linear radiating elements lie along axes which intersect to form an angle at the other of their ends.

11. A subsurface communications antenna comprising:

first and second insulated matched load terminated linear radiating elements positioned normal to and under the surface, the elements being spaced apart by distance D, the useful radiating element length L being approximated by B D/a when B B a; a and a,L l, where a and a are the attenuation constants of the radiating element and propagation medium respectively, 8 and B are the phase constants of the radiating element and propagation medium respectively;

means for mutually exciting the radiating elements; and

means for matching the phase velocity of an electromagnetic wave along the radiating elements to the phase velocity of a wave propagating in a medium external to the elements.

12. A subsurface communications antenna according to claim 11, wherein:

each radiating element propagation constant k, is related to the attenuation constant a and phase constant B by k B ja the spacing between the radiating elements being such that [kD] S )t /2, where A is the wavelength of the electromagnetic wave in the propagation medium.

13. A subsurface communications antenna comprising:

first and second insulated matched load terminated linear radiating elements spaced apart under the surface within the refracting cone at the surface interface, the elements being sufficiently close to provide phase reinforcement of waves propagating toward their matched load ends;

means for mutually exciting the radiating elements; and

a first and second plurality of inductive impedances interconnecting portions of each element at periodic intervals along their extent.

14. A subsurface communications antenna comprising:

a first and second pair of insulated linear radiating elements spaced apart and defining a plane normal to and positioned under the surface within the refracting cone at the surface interface, the surface ends of each element being terminated in a corresponding matched load the planes of the pairs being in spaced quadrature to define an om nidirectional beam pattern;

means for exciting the first radiating element pair in phased quadrature to the second radiating pair; and

means for matching the phase velocity of an electromagnetic wave propagating along each radiating element to the phase velocity of an electromagnetic wave propagating in a medium external to the radiating element.

UNrrEn STATES PATENT OFFECE @ETEFEQATE FQQRREQ'MQN Patent No. 3,680,133 Dated July 25, 1972 Inventofls) Carson K,H. Tsao, Joseph T, deBettenco'urt G Howard J Rowland It is certified that error appears in the above-identified patent and that said Letters Patent are hereby eorrected as shown below:

Before the first line insert The invention herein described was made in the course of or under a contract or subcontract thereunder, (or grant) with the Department of E eiense.

Signed and sealed this 5th day of February 1974.

(SEAL) Attest:

EDWARD M.FLETCHE R,JR. RENI? D. TEG'iMEEE'ER attesting Officer Acting Commissioner of Patents FORM F G- 1050 (16:59)

UsC'OMM-DC BOMB-P69 us. eovznnmsu'r PRINTING OFFICE 191s o--sse-au 

1. A subsurface antenna for transmission and reception of electromagnetic wave energy comprising: first and second insulated matched load terminated radiating elements positioned under the surface in a generally vertical position, the distance between the radiating elements is such as to provide phase reinforcement; means for mutually exciting said radiating elements; and means for matching an electromagnetic wave propagating along said radiating elements to the phase velocity of an electromagnetic wave propagating in a medium external to the radiating elements.
 2. A subsurface antenna according to claim 1, wherein: said radiating elements lie along axes which intersect to form an angle at one of their ends.
 3. A subsurface antenna according to claim 2, wherein: said radiating elements are linear.
 4. A subsurface antenna according to claim 1, wherein: said radiating elements are linear.
 5. A subsurface communications antenna comprising: first and second insulated matched load terminated linear radiating elements positioned normal to and under the surface, the elements being spaced apart by distance D, the useful radiating element length L being approximated by B2D/a2 when B1 B2, a1<<a2, and a1L>>1, where a1 and a2 are the attenuation constants of the radiating element and propagation medium respectively, B1 and B2 are the phase constants of the radiating element and propagation medium respectively; means for mutually exciting the radiating elements; and means for matching the phase velocity of an electromagnetic wave propagating along said radiating elements to the phase velocity of an electromagnetic wave propagating in a medium external to the elements.
 6. A subsurface communications antenna according to claim 5, wherein: each radiating element propagation constant k1 is related to the attenuation constant a1 and phase constant B1 by k1 B1 - ja1, the spacing between the radiating elements being such that kD < or = lambda 2/2, where lambda 2 is the wavelength of the electromagnetic wave in the propagation medium.
 7. A subsurface antenna for transmission and reception comprising: first and second insulated matched load terminated linear radiating elements positioned under the surface, the radiating elements being spaced apart within the refracting cone at the surface interface; means for mutually exciting the radiating elements; and means for matching the phase velocity of an electromagnetic wave propagating along the radiating elements to the phase velocity of an electromagnetic wave propagating in a medium external to the radiating elements.
 8. A subsurface antenna for transmission and reception comprising: first and second insulated linear radiating elements spaced apart and defining a plane under the surface within the refracting cone of the surface interface, each element having one end near the surface terminated in a corresponding matched load; means for mutually exciting the radiating elements; and means for matching the phase velocity of an electromagnetic wave propagating along the radiating elements to the phase velocity of an electromagnetic wave propagating in a medium external to the elements.
 9. A subsurface antenna according to claim 8, wherein: the first and second insulated linear radiating elements are positioned normal to the surface.
 10. A subsurface antenna according to claim 8, wherein: the first and second insulated linear radiating elements lie along axes which intersect to form an angle at the other of their ends.
 11. A subsurface communications antenna comprising: first and second insulated matched load terminated linear radiating elements positioned normal to and under the surface, the elements being spaced apart by distance D, the useful radiating element length L being approximated by B2D/a2 when B1 B2, a1<<a2, and a1L>>1, where a1 and a2 are the attenuation constants of the radiating element and propagation medium respectively, B1 and B2 are the phase constants of the radiating element and propagation medium respectively; means for mutually exciting the radiating elements; and means for matching the phase velocity of an electromagnetic wave along the radiating elements to the phase velocity of a wave propagating in a medium external to the elements.
 12. A subsurface communications antenna according to claim 11, wherein: each radiating element propagation constant k1 is related to the attenuation constant a1 and phase constant B1 by k1 B1 - ja1, the spacing between the radiating elements being such that kD < or = lambda 2/2, where lambda 2 is the wavelength of the electromagnetic wave in the propagation medium.
 13. A subsurfacE communications antenna comprising: first and second insulated matched load terminated linear radiating elements spaced apart under the surface within the refracting cone at the surface interface, the elements being sufficiently close to provide phase reinforcement of waves propagating toward their matched load ends; means for mutually exciting the radiating elements; and a first and second plurality of inductive impedances interconnecting portions of each element at periodic intervals along their extent.
 14. A subsurface communications antenna comprising: a first and second pair of insulated linear radiating elements spaced apart and defining a plane normal to and positioned under the surface within the refracting cone at the surface interface, the surface ends of each element being terminated in a corresponding matched load, the planes of the pairs being in spaced quadrature to define an omnidirectional beam pattern; means for exciting the first radiating element pair in phased quadrature to the second radiating pair; and means for matching the phase velocity of an electromagnetic wave propagating along each radiating element to the phase velocity of an electromagnetic wave propagating in a medium external to the radiating element. 